Primary and secondary power supplies are often used in tandem to power various forms of application circuitry (e.g., timekeeping). Inputs for primary power supplies have typically been electrically coupled to system power sources and inputs for secondary power supplies have been electrically coupled to back-up power supplies (e.g., batteries), neither of which were rechargeable (e.g., rechargeable batteries). The power management circuitry used to control power supplies have not allowed convenient mechanisms or methods that allow users to access and/or recharge batteries that are used as a power supply. As a result, a user's choice has been rather limited. The user could use only the system power supply (e.g., for the first power source) and non-rechargeable batteries for the primary and secondary power supplies. Alternatively, if the user insisted on rechargeable batteries, external charging circuitry was required, because necessary circuitry (if it existed at all) could not be easily integrated into an integrated circuit. As referenced above, a reliable power supply is an especially important consideration in time-keeping applications, because users generally want to perpetually save, back-up, and otherwise preserve timekeeping information (e.g., what time is it?). However, it is important in other applications as well.
The use of multiple power supplies in systems can introduce other complications to integrated circuits, especially CMOS integrated circuits. CMOS circuits powered by two different supply levels cannot be directly connected together because of the diodes that typically exist in CMOS circuits for processing reasons and for ESD protection. In addition, power consumption may increase when connecting circuits that are powered by power supplies having differing power supply levels.
More specifically, two situations generally apply: a circuit being supplied by a higher supply driving a circuit being powered by a lower supply and a circuit being supplied by a lower supply level driving a circuit being powered by a higher supply. In the first situation, the circuit powered by a voltage supply having the lower supply level generally has an input that transitions above the voltage level corresponding to the power supply powering it, because it is being driven by the circuit with the higher supply. Since a diode typically exists between the input of the lower supply level circuit and its supply voltage, any transition more than a diode drop above its supply will forward bias this diode and must not be allowed. Not only could this cause a latch-up problem within the circuit but the input pin would actually begin to supply the current for the operation of the lower supply level circuit.
The second situation addresses a power issue which has become very important over the last several years. Input signals from circuits that have lower supply levels than those of the circuits being driven may not transition high enough to stop the flow of current through input buffers of the higher supply level circuits. This will increase the power used by the higher supply circuit.
In addition, adding features to existing product lines introduces additional factors, such as compatibility issues. New generations of parts are preferably compatible with previous generations of parts. In addition, combing additional features, such as write protection, introduces additional requirements. This provides functionality to a broader application base with a single part. As a result, new features, such as those discussed above, need to be able to be added to or integrated with existing systems without altering the basic functionality or operation of previous generations of electrical devices (e.g., integrated circuits). For instance, when there are a limited number of pins, one cannot simply add an extra pin to invoke or activate the newly added feature. Thus, it complicates the addition of new features without affecting the basic functionality.